Yemeni officials said the killing bore the hallmarks of an al-Qaida attack, but it was too early to determine whether the group was behind it. The assassination resembles other suspected al-Qaida attacks recently that have targeted Yemeni intelligence, military and security officials. The attacks are believed to be in retaliation for a military offensive by Yemen's U.S.-backed government against the terror group's branch in the country.

Yemen-based Al-Qaida in the Arabian Peninsula (AQAP), which Washington considers the most dangerous offshoot of the terror network, has called for attacks on U.S. embassies in a bid to exploit the anti-American sentiment that has swept the Middle East and other parts of the Muslim world in the past month over an anti-Islam film produced in the United States.

Initially, the film was linked to an attack on the U.S. Consulate in the Libyan city of Benghazi on September 11 which left four Americans dead including U.S. Ambassador to Libya Chris Stevens. U.S. officials said later the attack was not linked to the video.

Still AQAP praised the killing of U.S. diplomats in Libya, describing it as "the best example" for those attacking embassies to follow.

Yemeni officials identified the embassy security official as Qassem Aqlani, in his fifties. He was on his way to work when a gunman on a motorcycle opened fire at him and fled the scene. The attack was in western Sanaa, close to Aqlani's home, while the embassy is located in eastern Sanaa.

Aqlani had been working for the U.S. Embassy for nearly 20 years, said the officials who spoke condition of anonymity because they were not authorized to speak to the media. Most recently, he was in charge of investigating a Sept. 12 assault on the U.S. Embassy by angry Yemeni protesters over the anti-Islam film. Protesters stormed the embassy and set fire to a U.S. flag before government forces dispersed them with tear gas. That attack came one day after the killings of the Americans in Benghazi.

AQAP had taken advantage of a security and political vacuum created by last year Arab Spring-inspired uprising and seized territories and cities in the south. The government-led offensive has pushed the militants out to mountainous areas from where they have been staging suicide attacks and assassinations inside cities.

Two weeks ago, a top intelligence official, Col. Abdullah al-Ashwal, was also killed in a drive-by shooting in Sanaa.